Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
Note that there is a subtle difference in Nix that causes
nixos-rebuild to work and NixOps to fail:
$ nix-instantiate '<nixos>' -A config.system.nixosVersion --eval-only
"13.10pre34915.50f4822"
$ nix-instantiate '<nixos/default.nix>' -A config.system.nixosVersion --eval-only
error: opening file `/nix/var/nix/profiles/per-user/root/channels/nixos/.version': No such file or directory
FixesNixOS/nixops#145.